Androgens are known to mediate their effects through the androgen receptor (AR). Androgens play a role in a wide range of developmental and physiological responses, for example, male sexual differentiation, maintenance of spermatogenesis, and male gonadotropin regulation (R. K. Ross, G. A. Coetzee, C. L. Pearce, J. K. Reichardt, P. Bretsky, L. N. Kolonel, B. E. Henderson, E. Lander, D. Altshuler & G. Daley, Eur Ural 35, 355-361 (1999); A. A. Thomson, Reproduction 121, 187-195 (2001); N. Tanji, K. Aoki & M. Yokoyama, Arch Androl 47, 1-7 (2001)). Also, androgens are associated with the development of prostate carcinogenesis. Induction of prostatic carcinogenesis in rodent models has been associated with androgens (R. L. Noble, Cancer Res 37, 1929-1933 (1977); R. L. Noble, Oncology 34, 138-141 (1977)) and men receiving androgens in the form of anabolic steroids are reported to have a higher incidence of prostate cancer (J. T. Roberts & D. M. Essenhigh, Lancet 2, 742 (1986); J. A. Jackson, J. Waxman & A. M. Spiekerman, Arch Intern Med 149, 2365-2366 (1989); P. D. Guinan, W. Sadoughi, H. Alsheik, R. J. Ablin, D. Alrenga & I. M. Bush, Am J Surg 131, 599-600 (1976)). Furthermore, prostate cancer does not develop if humans or dogs are castrated before puberty (J. D. Wilson & C. Roehrborn, J Clin Endocrinol Metab 84, 4324-4331 (1999); G. Wilding, Cancer SUM 14, 113-130 (1992)). Castration of adult males causes involution of the prostate and apoptosis of prostatic epithelium (E. M. Bruckheimer & N. Kyprianou, Cell Tissue Res 301, 153-162 (2000); J. T. Isaacs, Prostate 5, 545-557 (1984)). This dependency on androgens provides the underlying rationale for treating prostate cancer with chemical or surgical castration (i.e. androgen ablation).
Prostate cancer is the second leading cause of male cancer-related death in Western countries (Damber, J. E. and Aus, G. Lancet (2008) 371:1710-1721). Numerous studies have shown that the androgen receptor (AR) is central not only to the development of prostate cancer, but also the progression of the disease to the castration resistance state (Taplin, M. E. et al. J. Clin. Oncol. (2003) 21:2673-8; and Tilley, W. D. et al. Cancer Res. (1994) 54:4096-4102). Thus, effective inhibition of human AR remains one of the most effective therapeutic approaches to the treatment of advanced, metastatic prostate cancer.
The AR possesses a modular organization characteristic of all nuclear receptors. It is comprised of an N-terminal domain (NTD), a central DNA binding domain (DBD), a short hinge region, and C-terminal domain that contains a hormone ligand binding pocket (the ligand binding domain, which also comprises the hormone binding site (HBS)) and the Activation Function-2 (AF2) site (Gao, W. Q. et al. Chem. Rev. (2005) 105:3352-3370). The latter represents a hydrophobic groove on the AR surface which is flanked with regions of positive and negative charges—“charge clamps” that are significant for binding AR activation factors (Zhou, X. E. et al. J. Biol. Chem. (2010) 285:9161-9171). Recent studies have identified a novel site on the AR called Binding Function 3 (BF3) that is involved into AR transcriptional activity. When the AR translocates into the nucleus, the DBD dimerizes and binds to androgen response elements (AREs), and thus induces transcription, which is an essential process of AR transcription for both wild-type AR and AR splice variants. Importantly, the crystal structure of AR DBD dimer binding to AREs is available, which suggests the possibility and tractability to identify small-molecule inhibitors with novel mechanisms by targeting AR DBD through a rational, structure-based drug design. Moreover, as all mechanisms of resistance studied to date still involve the binding of AR to DNA, and the DBD exists in both wild-type AR and splice variants, targeting DBD represents a new approach to overcome resistance.
The activation of AR follows a well characterized pathway: in the cytoplasm, the receptor is associated with chaperone proteins that maintain agonist binding conformation of the AR (Georget, V. et al. Biochemistry (2002) 41:11824-11831). Upon binding of an androgen, the AR undergoes a series of conformational changes, disassociation from chaperones, dimerization and translocation into the nucleus (Fang, Y. F. et al. J. Biol. Chem. (1996) 271:28697-28702; and Wong, C. I. et al. J. Biol. Chem. (1993) 268:19004-19012) where it further interacts with co-activator proteins at the AF2 site (Zhou, X. E. et al. J. Biol. Chem. (2010) 285:9161-9171). This event triggers the recruitment of RNA polymerase II and other factors to form a functional transcriptional complex with the AR.
Notably, the current anti-androgens such as bicalutamide, flutamide, nilutamide and MDV3100, all target this particular process. These anti-androgens act by binding to the AR ligand binding site. Thus, by preventing androgens from binding they also prevent conformational changes of the receptor that are necessary for co-activator interactions. While treatment with these AR inhibitors can initially suppress the prostate cancer growth, long term hormone therapy becomes progressively less effective (Taplin, M. E. et al. J. Clin. Oncol. (2003) 21:2673-8; and Tilley, W. D. et al. Cancer Res. (1994) 54:4096-4102). There is thus a significant need for additional compounds targeting AR for treatment of cancer.
Androgens also play a role in female cancers. One example is ovarian cancer where elevated levels of androgens are associated with an increased risk of developing ovarian cancer (K. J. Helzlsouer, et al., JAMA 274, 1926-1930 (1995); R. J. Edmondson, et al, Br J Cancer 86, 879-885 (2002)). The AR has been detected in a majority of ovarian cancers (H. A. Risch, J Natl Cancer Inst 90, 1774-1786 (1998); B. R. Rao & B. J. Slotman, Endocr Rev 12, 14-26 (1991); G. M. Clinton & W. Hua, Crit Rev Oncol Hematol 25, 1-9 (1997)), whereas estrogen receptor-alpha (ERa) and the progesterone receptor are detected in less than 50% of ovarian tumors.